Because of its high temperature strength and creep resistance and low thermal expansion as well as its excellent oxidation and corrosion resistance, silicon nitride (Si.sub.3 N.sub.4) has been used for many years to make critically engineered parts such as gas turbine blades.
Generally, it has been the practice to form silicon nitride Parts by "reaction bonding" or by "hot pressing" or by a pressureless sintering process. Reaction bonding comprises compacting silicon powder of commonly less than 400 mesh into the part commonly at ambient temperature and then exposing the part to molecular nitrogen at about 1400.degree. C. for a period of time sufficient to convert the silicon to silicon nitride such as disclosed in U.S. Pat. No. 4,235,857, the disclosure of which is incorporated herein by reference. Such is also reviewed by A. J. Moulson in an article titled "Review Reaction-Bonded Silicon Nitride: Its Formation and Properties," Journal-Materials Science, 14, (1979) 1017-1051 and by Mangels in an article titled "Effect of Rate-Controlled Nitriding and Nitriding Atmospheres on the Formation of Reaction-Bonded Si.sub.3 N.sub.4 ", Ceramic Bulletin, Volume 60, No. 6 (1981), 613 in which he also describes benefits derived by using a nitriding gas mixture of nitrogen with minor amounts of hydrogen and helium. The use of a combined nitrogen-hydrogen nitriding gas in the reaction bonding of Si.sub.3 N.sub.4 is described by Shaw and Zeleznik in an article titled "Thermodynamics of Silicon Nitridation: Effect of Hydrogen", Communications of the American Ceramic Society, November 1982, C180-C181 and the effect of temperature and nitrogen pressure on the kinetics of silicon nitridation along with the need for an activating agent such as iron is described by Atkinson, Moulson and Roberts in an article titled "Nitridation of High-Purity Silicon", Journal American Ceramic Society, Volume 59, No. 7-8, 285-289.
Hot pressing involves pressing alpha-silicon nitride powder into the part at sintering temperatures of about 1700.degree. C. to about 2200.degree. C. for a prescribed period of time. In the "hot pressing" process, it has further been the practice to add "densification or sintering aids" to the silicon nitride powder to reduce porosity and improve strength. Pressureless sintering involves sintering of alpha-phase silicon nitride powder at temperatures of above 1650-1800.degree. C. to which has been added "densification or sintering aids" similar to "hot pressing." Examples of "densification or sintering aids" include monovalent metal oxides such as Mg0, ZrO, Ni0, and divalent metal oxides such as Al.sub.2 0.sub.3, Cr.sub.2 0.sub.3 and y.sub.2 0.sub.3 such as disclosed in U.S. Pat. No. 3,950,464, the disclosure of which is incorporated herein by reference.
It is also common to include nitriding agents in the "reaction bonding" process but those presently known to be suitable is much more limited than the "densification or sintering aids" previously described and generally include iron oxide or nickel oxide and mixtures thereof at the present time.
It has also been common Practice to prepare the silicon or silicon nitride powder based compounds by mixing, commonly in a ball mill, with a wetting agent. Commonly such wetting agents have been members of the alcohol family such as tertiary alcohol disclosed in U.S. Pat. Nos. 3,991,166 and 3,992,497, the disclosures of which are incorporated herein by reference.
It has been less common to use water in preparing silicon or silicon nitride ceramic compounds but an example of the use of about 10% by weight water to fluidize a sinterable molding powder prior to freezing is disclosed in U.S. Pat. No. 2,869,215, the disclosure of which is incorporated herein by reference. U.S. Pat. No. 2,268,589, the disclosure of which is incorporated herein by reference, discloses an early use of water in forming a paste with activated silicon which is then fired at 1200.degree. C. to 1450.degree. C.
In addition to nitriding and densification agents, it has also been common practice to employ binders in the ceramic compounds to bind the silicon or silicon nitride powder particles together to enhance their forming into the parts being made. Examples of such binders include a mixture of butyl methacrylate and trichlorethylene disclosed in U.S. Pat. No. 3,819,786 in conjunction with silicon nitride powder and a blend of polyvinyl alcohol and water and a silicon carbide and berylium oxide blend disclosed in U.S. Pat. No. 3,205,080, the disclosures of which are incorporated herein by reference.
Up until the time of the present invention, it has been the further practice to nitride silicon powder by heating for long periods of time. An example of such is disclosed in U.S. Pat. No. 3,819,786, previously described, where a blend of silicon nitride powder and the binder mixture is heated in a stream of nitrogen from ambient to 1000.degree. C. at 50.degree. C./hr and then held under static nitrogen for 20 hours at 1350.degree. C. and 10 hours at 1450.degree. C. with the total time more than thirty hours long.
An example of a compound heating schedule for nitriding a mixture of silicon and silicon carbide powder is disclosed in U.S. Pat. No. 3,222,438, the disclosure of which is incorporated herein by reference, where the mixture is first compacted into a green compact and then heated in an atmosphere of nitrogen at a temperature of 1250.degree. C. for 16 hours and then at 1450.degree. C. for 3-4 hours where the first stage heating is conducted to pre-sinter the compound so that it doesn't melt at the 1450.degree. C. temperature since the melting point of silicon is about 1420.degree. C.
Finally, another example of a compound heating schedule for sintering a silicon nitride and Mg0 powder mixture is disclosed in U.S. Pat. No. 3,992,497, the disclosure of which is incorporated herein by reference, where the mixture is formed into a compact and then first heated at a temperature of about 600.degree. C. for about 60 minutes to remove volatiles and then the temperature is increased to a temperature between 1500.degree. C. and 1700.degree. C. at a rate of climb above 1450.degree. C. being about 15.degree. C. to 200.degree. C. per minute and holding it at that temperature for a period between about 5 and 30 minutes and more specifically teaching that the heating rate is immaterial until the temperature of about 1450.degree. C. is attained.
Contrary to the above, the process of the present invention, although employing known binders and nitriding agents, utilizes relatively large amounts of water under prescribed conditions in providing a homogeneous slurry of silicon powder and at least one nitriding agent which is subsequently processed and formed into green stock which is then nitrided with a unique compound heating schedule incorporating a multi-component nitriding atmosphere to provide high quality silicon nitride articles in a rapid and economical manner.